Download Amazon appstore on Android after Nougat.
In this video I demonstrate how to download the amazon appstore on my Sansung Note 8 running Android Nougat 7.1.1. Further in my description I explain how to install it.
On this website it describes how to install the Amazon Appstore https://www.google.com/amp/s/forums.androidcentral.com/samsung-galaxy-note-5/597560-amazon-underground-app-store.html%3Famp
Tab Guy explains it this way, and it worked for me.
I know this sounds odd but you have an app running that has the ability “pop over” the screen. When that happens the Install button on a side loaded app won’t work. I think Google does this so that a rogue app can’t install another app on its’ own. In my case I have an app called Popup Notification which will put notifications on top of the screen. Once I killed that app the “Install” button works. If you don’t know which app is doing this you can reboot and then install. But you must be quick before the pop over app gets loaded.
source